1. Field of the Invention
The present invention relates generally to radio-frequency identification (RFID) technology, and in particular relates to RFID-based systems and methods for detecting that a condition has occurred in an RFID tag even when the RFID tag is not powered, by storing and managing power in RFID tags.
2. Technical Background
Radio-frequency identification (RFID) is a remote recognition technique that utilizes RFID tags having information stored therein, usually in an integrated circuit (IC). The stored information is retrievable via RF communication between the RFID tag and an RFID tag reader. Certain RFID systems utilize hand-held RFID readers that when brought sufficiently close to an RFID tag are able to read an RFID tag signal either emitted by or backscattered from the tag. RFID systems are used for a variety of applications, including inventory management and product tracking in a number of different industries, as well as in libraries and hospitals.
RFID tags generally come in three varieties: passive, semi-passive, and active. Passive RFID tags have no energy or power source of their own and operate by harvesting energy from the RF signal (field) generated by the RFID-tag reader. Passive tags communicate back to the reader by modulating and back-scattering the RF signal from the RF reader. Semi-passive RFID tags communicate to the reader in the same way via modulation of the back-scattered reader RF signal, but they do not rely on harvesting energy from the reader field to power the RFID tag IC. Instead, semi-passive tags generally have their own power source, usually in the form of one or more batteries. Since the amount of power harvested by a passive tag usually limits its maximum distance from the reader antenna, semi-passive RFID tags usually have significantly greater read ranges than passive tags. Active tags also have a power source such as a battery that not only powers the RFID tag IC but that can also actively generate and transmits radiation to the RFID reader.
RFID tags can be designed to operate at different RF frequencies. At low frequencies (e.g., 100-130 KHz s) RFID tags often communicate via mutual inductance coupling between an RFID-reader coil antenna and an RFID-tag coil antenna. At these frequencies, the RFID reader's RF signal is not strongly absorbed by water. Since the user's hand is primarily composed of water, this means that at low RF frequencies the RF signal can penetrate the user's hand and enable two-way communication between the RFID tag and the RFID reader.
This low frequency inductance coupling approach is practical as long as the distance between the tag and the reader is a fraction of the wavelength of the RF signal. In typical low frequency RFID tag applications, the required separation between the RFID tag and the RFID reader (i.e., the “read distance”) must be less than 1 m. This small separation is not suitable for many applications involving the RFID identification of hand-held items. In particular, the separation distance between RFID tags and RFID readers in typical item-identification applications within telecommunications data centers is expected to be 1 m to 3 m. Therefore, the low frequency RFID tag solution is not a practical approach for this and other such applications.
RFID tags designed to operate at higher frequencies (e.g., ultra-high frequencies of 900 MHz or greater) typically operate by the RFID tag capturing far-field radiation from the RFID reader antenna transmission using a local monopole, dipole or modified dipole antenna (e.g., a “squiggle antenna”).
The electric signal formed by the RFID tag antenna is processed by a rectifier circuit in the RFID tag's IC chip, yielding energy that powers the rest of the IC chip and enables the IC chip to transmit a return signal to the RFID reader. As mentioned above, communication with the RFID reader is often via backscatter modulation of the RFID reader signal rather than by independent RF signal transmission from the RFID tag.
Ultra-high-frequency RFID tags can communicate with RFID readers at much greater read distances (e.g., 5 to 10 m) than low frequency RFID tags (1 m or less). Ultra-high-frequency RFID tags are thus better suited for applications involving the RFID identification of hand-held items.
A problem with using ultra-high frequency RFID tags for the identification of hand-held items arises due to the strong absorption of high-frequency RF signal power by water. When a user places their hand around an item with an RFID tag just prior to engaging, water in the user's hand attenuates the reader's RF signal when the hand blocks the RF communication path. Even in cases where the user's hand only partially blocks the RF communication path, it can still significantly reduce the RF signal strength. In this situation, the RFID tag may not receive sufficient energy to power its internal circuitry. Likewise, the RFID tag's response to the RFID reader may be impeded by the presence of the user's hand.
In the case where the RFID tag does not receive adequate power to operate, no signal is communicated to the RFID tag reader. When the user removes their hand from the item and disengages the switch, then the RFID tag once again receives RF power from the RFID reader and powers up. An approach is described that enables the RFID tag reader to detect when an event has occurs at the RFID tag during periods when the power to the RFID tag has been interrupted.